Observations and simulations of a non‐stationary coastal atmospheric boundary layer

This paper studies the turbulent and mesoscale structures of a coastal marine atmospheric boundary layer in the southern Baltic Sea. Data from a field experiment are examined for a case during autumn, and this case is subsequently modelled with a mesoscale numerical model. During this day, the marine atmospheric boundary layer develops from a well‐mixed cloud‐capped state into a stably stratified, sheared boundary layer. This development is due to the dissipation of the cloud layer by radiation processes and by synoptic‐scale subsidence. The boundary layer becomes non‐stationary, and spectral analysis shows that the turbulence structure deviates from the ideal steady‐state homogeneous boundary‐layer turbulence, which forms the basis for most turbulence closures in numerical models. It is shown that the subsiding boundary‐layer top and the increasing stability alter the spatial scale of the turbulent eddies; they become vertically distorted and smaller than those expected during steady‐state conditions. In spite of these deviations, the turbulence statistics are sufficiently well behaved for simplified higher‐order closure modelling to be applicable. When the mixed layer collapses due to the dissipating clouds, the transition triggers an inertial oscillation and a non‐classical low‐level jet forms. At the same time, the changes in depth and stability of the marine boundary layer are perturbed by the mesoscale interaction between the flow and the coastal terrain. Thus, the upstream terrain increasingly blocks the flow, so that the inertia) jet forms only for the part of the flow that overcomes the terrain obstacle. A numerical simulation of the case reveals the primary mechanisms, and several sensitivity simulations, wherein the terrain and the time‐evolution of the forcing are manipulated, confirm the hypotheses proposed as causing the observed (low structure.